In my twenties/thirties I used Henna. It looked quite good, but I got fed up with it. I'm a natural blond, but going grey. What I like the best, is to get thin stripes put in, leaving a bit of your own color. I like to get three different color stripes. That way, you can let your hair grow out without getting that awful edge you will get with a full color. Nothing looks more horrible! It's expensive, but it will last for at least 6 months.

Another option is to get a "shampoo color" that you leave in for 4-5 min. It washes out after 7-8 washes, depending of which shampoo you use, but it doesn't leave that awful edge. Unfortunately, I can't get it in Copenhagen, but when I'm in England I buy it. Superdrug has it. Get one close to your own color, though.